1 Field of the Invention
The present invention relates to an amplifier with digitally controlled gain and its application to a circuit for processing signals output by a read head of an optical disc reader apparatus.
2 Description of the Related Art
Referring to FIG. 1, information recorded on an optical disc 1 is read, in a manner which is known per se, by projecting a light beam 3 output by a laser diode 2 onto the reflective surface of the disc. The reflected beam 4 is detected by photodiodes A, B, C, D, E and F, the signals output by these photodiodes being used, inter alia, to decode the information stored on the optical disc. The photodiodes A to F, as well as the laser diode 2, form part of an optical and mechanical assembly 5 referred to as the read head of the apparatus, or optical pick-up, which does not form the subject-matter of the present invention and will not be described in further detail.
The signals output by the photodiodes A and C are added before being processed. The same is true of the signals output by the photodiodes B and D. Four signals S0 to S3 thus leave the read head 5 to be processed and decoded by a suitable processing circuit. In the case in point, the signals S0 to S3 correspond to the currents flowing through the photodiodes. These currents vary as a function of a certain number of factors, such as the reflectivity of the surface of the disc (depending on its state of cleanliness) or the power of the emitting laser diode 2.
Fixed-gain current/current amplifiers 10 to 13 receive the signals S0 to S3 and are connected to the four inputs E0 to E3 of a multiplexer 15. The output of the multiplexer 15 is connected to a resistor 16, which converts the current into voltage, and to the input of analogue/digital converter 18 which delivers a digital signal encoded using six bits to a digital circuit 25 for processing and decoding the signals emitted by the photodiodes.
In order to make maximum use of the dynamic range of the analogue/digital converter 18, and thus to deliver high-quality signals to the circuit 25, it is necessary for the signal entering the converter to have a sufficient amplitude. However, this amplitude depends directly on the current which is picked up by the photodiodes A to F. Depending on the type of read head which is used, the amplitude of the signal received by the analogue/digital converter 18 can vary from one to four times. These large variations are corrected by modifying the value of the resistor 16 as a function of the type of read head used. To that end, when the circuit for processing the signals output by the read head 5 is produced in integrated circuit form, a pin is provided on the circuit in order to make it possible to connect an external resistor 16 of suitable value to the read head.
However, the value of the resistor 16 is fixed for each given type of read head and does not make it possible to correct smaller but all the same significant variations in the current output by the photodiodes, which variations are due to the surface condition of the optical disc 1, which is not uniform (fingerprints, etc.) or the conditions under which the apparatus is being used (temperature, background light, etc.).
To solve this problem, it has been proposed in the prior art to modify the current flowing through the laser diode 2 as a function of the digital signal which is obtained at the output of the analogue/digital converter 18, so as to correct a decrease in the current output by the photodiodes by increasing the current in the laser diode, and vice versa. To do this, the digital signal output by the converter 18 is sent to the input of a digital/analogue converter 20, the analogue signal 8 resulting from the conversion being used to control the current in the laser diode 2.
However, this arrangement is unsatisfactory for the following reasons: on the one hand, it is necessary to add a digital/analogue converter 20, which increases the size of the assembly; on the other hand, the variations in the current in the laser diode lead to a significant reduction in its life.
It is furthermore known to use, in analogue/digital processing systems, a gain adjustment stage in order to match the amplitude of the incoming analogue signal to the dynamic range of the analogue/digital converters which are used in the processing system. FIGS. 2a to 2c illustrate different ways of adjusting the gain of an amplifier. In these figures, the amplifiers are voltage-controlled.
In FIG. 2a, an input signal V.sub.IN1 is applied to one of the gates of a two-gate MOSFET transistor 36 whose drain is connected to a supply V.sub.DD via a resistor 34 and whose source is earthed. The drain of the transistor 36 is also connected to an operational amplifier 37, set up in follower mode, whose output is connected to the input of an analogue/digital converter 38 which delivers a digital signal to a digital processing circuit 35. In order to adjust the amplitude of the signal entering the converter 38, the transconductance of the transistor 36 will be modified by applying, to the second gate of the transistor, a control voltage V.sub.COM1 coming from a return loop. The return loop is formed by a digital signal leaving the circuit 35 sent to a digital/analogue converter 39 whose output is connected to an operational amplifier 37', set up in follower mode, which delivers at an output the control voltage V.sub.COM1. The problem with this device is that the gain does not vary linearly as a function of the control voltage throughout the adjustment range of the gain.
In FIG. 2b, elements which are identical have the same references as in FIG. 2a. In this device, the input signal V.sub.IN2 is applied to a first non-inverting input of an operational amplifier 47 whose inverting input is earthed via a resistor 44 and which has a feedback loop connecting its output to its inverting input via a MOS transistor 46 set up in triode mode. The gain of the amplifier is modified by a control voltage V.sub.COM2 (taken from the output of the operational amplifier 37') which modifies the value of the resistor formed by the MOS transistor 46. This device, although it makes it possible to obtain a more linear variation in the gain, nevertheless has stability problems due to the feedback loop formed by the MOS transistor.
Furthermore, as in the case of FIG. 2a, the voltage for controlling the gain comes from a digital/analogue converter 39 which occupies a large amount of space, in particular when the assembly is produced in integrated circuit form.
In the third case in FIG. 2c, a network of resistors 56 is placed in the feedback path of an amplifier 57 which receives the input signal V.sub.IN3. The resistors 56 may or may not be connected in parallel using switching devices 59 which are controlled directly by a digital signal V.sub.COM3 output by the digital processing circuit 35. The gain is adjusted by connecting a greater or lesser number of resistors in the feedback path. This device makes it possible, advantageously in comparison with the former two, to do away with a digital/analogue converter 39. However, it also has stability problems due to the existence of the feedback loop.
Because of the various problems explained above, the controlled-gain amplifiers of the prior art cannot be used satisfactorily in the systems for processing the signals S0 to S3 in FIG. 1 in order to adjust the amplitude of the signals delivered to the mulitplexer 15 and therefore to the analogue/digital converter 18.